Millions of Americans were facing dangerously cold temperatures in the wake of a massive winter storm that whipped snow and ice across the country, knocking out power and paralysing transportation.
A frigid, potentially life-threatening Arctic air mass threatened to delay clean-up as municipalities from New Mexico to Maine tried to dig out following the storm, which dropped a cocktail of heavy snow and wind along with freezing rain and sleet.
A compilation of local media reports tallied at least 21 storm-related deaths.
Over 780,000 customers remained without electricity, the Poweroutage.com tracking site showed.
Tennessee, Texas, Mississippi and Louisiana, southern states unaccustomed to intense winter weather and the cold that is forecast to continue for much of the next week, were especially impacted.
Approximately 190 million people in the United States were under some form of extreme cold alert, the National Weather Service (NWS) told AFP.
Areas across 20 states received at least 30cm of snow and in many cases far more.
The NWS said New Mexico's Bonito Lake accumulated the highest US total over the weekend with 78cm.
New York's Central Park received 29cm, breaking a same-day snowfall record from 1905.
NWS meteorologist Allison Santorelli told AFP the agency was seeing widespread reports of significant ice accumulation, including in places unaccustomed to severe winter weather.
She said this storm recovery was particularly challenging because so many states were impacted meaning northern states with more resources were unable to share their equipment and resources with less-prepared southern areas.
Watch: Wind and snow swirling in New York
"A lot of those locations don't have the means or the resources to clean up after these events," she said.
"We're particularly concerned about the folks in those areas that are without power right now."
Dave Radell, an NWS meteorologist based in New York, told AFP that the character of this storm's snow was "very dry" and "fluffy," meaning the wind could lash it around with ease, impeding roadway-clearing efforts and visibility.
"That makes it even more challenging," he said.
Excruciating cold
The snowfall and biting icy pellets that pummeled cities saw impassable roads, cancelled buses and trains along with grounded flights, thousands of departures and arrivals were scrapped over the weekend, as concern turned to the hazardous temperatures set to linger for the better part of a week.
The brutal storm system was the result of a stretched polar vortex, an Arctic region of cold, low-pressure air that normally forms a relatively compact, circular system but sometimes morphs into a more oval shape, sending cold air pouring across North America.
Scientists say the increasing frequency of such disruptions may be linked to climate change, though the debate is not settled and natural variability plays a role.
People in the Great Lakes region woke up to extreme temperatures that could cause frostbite on exposed skin within minutes, in parts of Minnesota and Wisconsin, the NWS reported early morning temperatures as low as -30c.
Forecasted windchills in those areas that make the apparent temperature even more frigid could hit as low as -45.6c, the weather authority said.
As far south as the Gulf Coast was expected to see freezing temperatures each night into the end of the week.
At least 20 states and Washington DC were under states of emergency in order to deploy emergency personnel and resources.
Many cities had opened warming centres for those without shelter or without power to take refuge, and authorities across the country were continuing to encourage residents who could to stay home.
